1. Field of the Invention
The present invention relates to an automatic transmission apparatus and a straddle-type vehicle including the automatic transmission apparatus.
2. Description of the Related Art
Conventionally, straddle-type vehicles equipped with a multi-geared type transmission mechanism having a dog clutch are known. In recent years, straddle-type vehicles configured to perform gear shifting of the transmission mechanism using an actuator such as an electric motor have also become known. These types of straddle-type vehicles can eliminate the shifting operations by the rider and reduce the burden on the rider resulting from the shifting operations.
The multi-geared type transmission mechanism having a dog clutch is provided with a movable gear that can be moved in axial directions of a main shaft by a shift fork and a stationary gear that is immovable in the axial directions of the main shaft. Each of the movable gear and the stationary gear has an engaging portion including, for example, a protrusion or a recess. The protrusion and the recess define the dog clutch. When the engaging portion of the movable gear and the engaging portion of the stationary gear engage with each other, the movable gear and the stationary gear come into mesh with each other, and the movable gear and the stationary gear rotate integrally with each other. Upon gear shifting, the movable gear moves in one axial direction of the main shaft. In association with this movement, the engaging portion of the movable gear separates from the engaging portion of the stationary gear, which has engaged with the engaging portion of the movable gear, and another engaging portion of the movable gear comes into engagement with another engaging portion of the stationary gear. In the following description, the separation of an engaging portion (e.g., a protrusion) of the movable gear from an engaging portion (e.g., a recess) of the stationary gear is referred to as “dog disengagement”. On the other hand, the engaging of an engaging portion of the movable gear with an engaging portion of the stationary gear is referred to as “dog engagement”.
At the time of the dog engagement, it is possible that a clunking noise may occur. For example, the clunking noise occurs when the tip of the engaging portion of the movable gear hits the bottom part of the engaging portion of the stationary gear. When the moving speed of the movable gear is higher, this clunking noise tends to become louder. Accordingly, in order to reduce the clunking noise, it is conceivable to control the actuator so as to decrease the moving speed of the movable gear.
Japanese Unexamined Patent Application Publication JP 2010-078117 discloses a technique for suppressing the clunking noise in a transmission control apparatus provided with an electric motor for rotating a shift drum. In the above transmission control apparatus, the shift drum is rotated by duty-controlling the electric motor to perform the dog disengagement and the dog engagement. In the above transmission control apparatus, a control process of reducing a duty ratio is performed after the dog disengagement. More specifically, the higher the temperature of the lubricating oil for the shift drum, the smaller the duty ratio that is set. In addition, before the completion of the dog engagement, the duty ratio is set to 0% (i.e., the motor output is set to zero) so that the dog engagement is completed using only the rotary inertia of the shift drum.
If the relative rotational speed between the movable gear and the stationary gear is high and the moving speed of the movable gear is low in the dog engagement, the engaging portion of the movable gear may be repelled away by the engaging portion of the stationary gear. At this time, this repelling of the engaging portion of the movable gear and the engaging portion of the stationary gear results in noise. Then, the engaging portion of the movable gear that has been repelled away again approaches the engaging portion of the stationary gear, but it is in many cases repelled away again, and the same event may be repeated. As a consequence, the engaging portion of the movable gear and the engaging portion of the stationary gear may come into contact repeatedly, causing periodic noise. The above-mentioned transmission control apparatus has a risk of causing such periodic noise.
Moreover, as described above, it is possible that the clunking noise may occur also at the time of the dog engagement between the movable gear and the stationary gear. The periodic noise and the clunking noise are unpleasant to the rider. For this reason, the periodic noise and the clunking noise need to be reduced at the same time.